Method of producing a lead frame with composite film attached, and use of the lead frame

ABSTRACT

Disclosed is a composite film, a method of producing a lead frame with the composite film attached, and a method of bonding a semiconductor chip to this lead frame having the composite film attached. The composite film includes a base film and an adhesive layer on one or both sides of the base film. The composite film has a thickness of T (μm), the base film has an edge tearing strength of R (kg/20 mm), the adhesive layer has a total thickness of A, and the base film has a thickness of B, T being related to R by a numerical formula R&gt;0.6T-8 when T≦60, or by a numerical formula R≧28 when T&gt;60, and A/B being 0.5 to 1.4.

This application is a Divisional application of prior application Ser.No. 09/421,002, filed Oct. 20, 1999, now U.S. Pat. No. 6,302,991, whichis a Divisional application of prior application Ser. No. 08/844,430,filed Apr. 18, 1997 now U.S. Pat. No. 5,998,020.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a composite film which has excellentpunching capability and is useful as a bonding element for semiconductorpackaging.

(b) Description of Related Art

In semiconductor packaging, a bonding element which is a composite filmproduced by coating one or both sides of a base film with adhesives isused for bonding chips to lead frames to form LOC (lead on chip) or COL(chip on lead) structures or window-tub structures, or for bonding innerleads to heat spreaders to form composite lead frames with heat sinksattached. The bonding element is generally applied to the lead frames bypunching out the bonding element from the composite film over the leadframes and pressing the bonding elements to required parts of the leadframes. However, the yield rate of the resulting lead frames withcomposite film attached is low since flashes are frequently formed atthe punched edges of the composite film.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a composite film whichis so excellent in punching capability as not to form flashes at thetime of punching and are useful as bonding film in semiconductorpackaging.

Another object of the present invention is to provide a lead frame withcomposite film bonded to the required parts of the lead frame bypunching the composite film of the present invention and is reliablewhen used in semiconductor packages.

As a result of studies in the relationship between the properties ofcomposite film and flashes, we have found that flashing can beeffectively prevented when there exists specific relation between thethickness T of composite film and the edge tearing strength R and havecompleted the present invention based on the finding.

That is, the present invention provides a composite film, comprising abase film and an adhesive layer on at least one side of the base film,the composite film having a thickness of T (μm), the base film having anedge tearing strength of R (kg/20 mm), the adhesive layer having a totalthickness of A, the base film having a thickness of B, T being relatedto R by a numerical formula R>0.6T-8 when T≦60, or by a numericalformula R≧28 when T>60, and A/B being 0.5 to 1.4.

The present invention further provides a lead frame with a compositefilm attached, comprising a lead frame body and a composite film appliedto the lead frame body, the composite film being punched out from thecomposite film of the present invention.

The present invention further provides a method of producing a leadframe with a composite film attached, comprising punching out acomposite film from the composite film of the present invention,pressing the punched out composite film to a lead frame body, with oneadhesive layer contacting a surface of the lead frame body.

Herein, the edge tearing strength of base film is measured according toJIS C 2318.

Flashes, which tend to be formed at the edges of punched composite filmcomprising a base film and an adhesive layer on at least one side of thebase film, can be extremely decreased by using a composite film whereinT and R are related to each other by the above numerical formula and theratio of the thickness A of the adhesive layer to the thickness B of thebase film, A/B, is within the above-described range.

THE PREFERRED EMBODIMENTS OF THE INVENTION

The composite film of the present invention is produced by applyingadhesives to one or both sides of a base film, followed by drying. Theadhesives applied to both sides of the base film may be identical withor different from each other.

The desirable adhesives are heat resistant adhesives which contain asmain components heat resistant thermoplastic resins and have a glasstransition temperature (Tg) of 150 to 350° C., a water absorption of 3%or less and a broadening length of 2 mm or less, and, therefore,polyimide adhesives and polyamide adhesives are suitable.

Herein, the term “polyimide” means not only polyimide but also any otherresin containing imide bonds, such as polyamideimide, polyesterimide orpolyetherimide.

If the glass transition temperature is beyond the above-described range,or the water absorption is more than 3% by weight, or the broadeninglength is longer than 2 mm, the resistance to reflow cracking ofpackages tends to be decreased.

The water absorption of the heat resistant adhesives is more preferably2.5% by weight or less, particularly preferably 2.0% by weight or less.The broadening length is more preferably 1 mm or less, particularlypreferably 0.5 mm or less.

In addition, fillers, such as ceramic powder, glass powder, silverpowder, copper powder, resin powder, gum powder and coupling agents mayalso be added to the heat resistant adhesives. The heat resistantadhesives may also be used after impregnating with it a base sheet, suchas glass fabric, aramid fabric or carbon fiber fabric.

Usable examples of the coupling agents include; vinylsilanes, such asvinyltriethoxysilane, vinyltrimethoxysilane andγ-methacryloxy-propyltrimethoxysilane; epoxysilanes, such asγ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldiethoxysilaneand β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; aminosilanes, such asγ-aminopropyltriethoxysilane, γ-aminopropyltrimethoxysilane andN-phenyl-γ-aminopropyltrimethoxysilane; mercaptosilanes, such asγ-mercaptopropyltrimethoxysilane; and coupling agents such as titanates,aluminum chelates and zircoaluminates.

Of these, silane coupling agents are preferred with epoxysilane couplingagents being particularly preferred.

Herein, “broadening length of an adhesive” defines a width by which afilm of the adhesive of 19×50 mm and 25 μm thick is broadened outperpendicularly from the middle of each side of the film by pressing thefilm at 350° C. at 3 MPa for one minute.

The base film to be used in the present invention, preferably, is a heatresistant film made of an engineering plastic, such as polyimide,polyamide, polysulfone, polyphenylenesulfide, polyetheretherketone orpolyarylate. The thickness of the base film is preferably 5 to 150 μm,more preferably 20 to 125 μm.

The glass transition temperature (Tg) of the heat resistant film ispreferably higher than the Tg of the adhesives to be used in the presentinvention, and is preferably 200° C. or higher, more preferably 250° C.or higher. The heat resistant film preferably has a water absorption of3% by weight or less, more preferably 2% by weight or less.

Therefore, polyimide film is suitable as the base film to be used in thepresent invention because of the high Tg, the low water absorption andthe low thermal expansion coefficient thereof. The particularlypreferred film has a Tg of 250° C. or higher, a water absorption of 2%by weight or less and a thermal expansion coefficient of 3×10⁻⁵/° C. orless.

To increase the adhesion force between the base film and the adhesives,it is preferable to surface treating the base film. The surfacetreatments applicable in the present invention are not limited andinclude, for example, chemical treatment, such as alkali treatment andsilane coupling agent treatment, physical treatment, such as sandblasting, plasma treatment or corona treatment, and it is possible toselect one or more treatments that are the most suitable for theadhesives used. In cases where heat resistant film is used as the basefilm and adhesive layers of heat resistant adhesive are applied thereto,chemical treatment or plasma treatment is particularly suitable.

The method of forming the adhesive layers on the base film is notparticularly limited, and an example of suitable methods comprisesapplying adhesive varnish to the base film and drying to removesolvents. The method of applying the adhesive varnish to the base filmis not particularly limited, and may be any one using a doctor blade, aknife coater or a dye coater. The application may also be performed bydipping the base film in adhesive varnish, but it may be difficult tocontrol the thickness of the adhesive layers.

For example, the adhesive varnish may be a varnish of a heat resistantresin, such as a heat resistant thermoplastic resin, or a heat resistantadhesive comprising mainly of a heat resistant resin, dissolved insolvents. Alternatively the adhesive varnish may be a varnish of a heatresistant resin precursor which can be converted into a heat resistantresin by heat treatment or the like following to coating, or an adhesivecomposition comprising mainly of the precursor, dissolved in solvents.Examples of the precursors are polyamic acids, which can be convertedinto polyimides by heat treatment.

When adhesive varnish applied to the base film is subjected to heattreatment for the removal of solvents or for conversion into imide,polyamic acid varnish and polyimide varnish are treated at differenttemperatures. Polyamic acid varnish is preferably treated at atemperature not lower than the Tg of polyimides to perform theconversion into the polyimides, while polyimide varnish may be treatedat any temperature sufficient to remove solvents.

After the adhesive layers are formed on the base film, it is preferableto conduct heat treatment at a temperature of 250° C. or higher for 1 to30 minutes to improve the adhesion force between the adhesive layers andthe base film.

The thickness of each adhesive layer formed on the base film ispreferably 5 to 50 μm, more preferably 10 to 30 μm.

The lead frame of the present invention comprises a lead frame body andcomposite film applied to the lead frame body, the composite film beingpunched out from the composite film of the present invention.

The lead frame bodies may have any structure and, for example, compriseinner lead portions to be connected to a semiconductor chip, outer leadportions to be connected to external circuits, and the composite film ofthe present invention is applied to predetermined portions of the leadframe bodies.

The lead frames with composite film attached may be produced by bondingthe composite film to lead frame bodies by punching the composite filmonto the lead frame bodies. Semiconductor packages produced by using thelead frames with composite film attached are excellent in reliability.

For example, the bonding of the composite film to lead frames may beefficiently performed by placing the composite film over a lead framebody, with one adhesive layer facing the lead frame, punching thecomposite film into strips by using punching metal molds, whichcontinuously press the punched out strips to predetermined portions ofthe underlying lead frame body at a pressure of 0.1 to 10 MPa for 0.1 to5 seconds, to bond them by their adhesive layers. The forms of thepunched out strips of the composite film vary, for example, depending onthe forms of chips, the locations of pads on chips or the designs oflead frames. At the time of punching, the lead frame body is generallyheated to a predetermined temperature, for example, 200 to 500° C. Thecomposite film to be punched may also be heated. If flashes are formedaround the punched strips, the flashes adhere to bonding areas of leadframes, to interfere wire bonding.

Hereinafter, the present invention will be described in detail referringto examples, but the scope of the invention is not limited by theexamples.

EXAMPLE 1

Each side of a polyimide film (Trade name: UPILEX-S, produced by UbeIndustries, Ltd., Tg>300° C., water absorption: 1.2% by weight, thermalexpansion coefficient: 1.6×10⁻⁵/° C.) having an edge tearing strength of35 kg/20 mm and a thickness of 25 μm was coated with a 17 μm thick layerof a polyamideimide adhesive (Trade name: HIMAL, produced by HitachiChemical Co., Ltd.) having a Tg of 185° C., a water absorption of 1.7%by weight and a broadening length of 0.05 mm, to produce a compositefilm of a total thickness of 59 μm.

The composite film was placed over a lead frame of 42 ally heated to400° C., and was then punched by using punching metal molds to formpunched out strips, which were then continuously pressed by the punchingmetal molds to leads of the underlying lead frame for one second at apressure of 3 MPa, to give a lead frame with composite film attached. Noflashes were observed by microscopic examination of the edges of thepunched composite film.

COMPARATIVE EXAMPLE 1

A composite film was produced in the same manner as in Example 1 withthe exception that a polyimide film (UPILEX-S) having an edge tearingstrength of 25 kg/20 mm and a thickness of 25 μm was used.

A lead frame with composite film attached was produced in the samemanner as in Example 1 with the exception that the composite film madeas above was used in place of the composite film of Example 1. Flasheswere observed by microscopic examination of the edges of the punchedcomposite film.

EXAMPLE 2

Each side of a polyimide film (UPILEX-S) having an edge tearing strengthof 30 kg/20 mm and a thickness of 50 μm was coated with a 25 μm thicklayer of the same polyamideimide adhesive as that used in Example 1, toproduce a composite film of a total thickness of 100 μm.

A lead frame with composite film attached was produced in the samemanner as in Example 1 with the exception that the composite film madeas above was used in place of the composite film of Example 1. Noflashes were observed by microscopic examination of the edges of thepunched composite film.

EXAMPLE 3

Each side of a polyimide film (UPILEX-S) having an edge tearing strengthof 30 kg/20 mm and a thickness of 25 μm was coated with a 17 μm thicklayer of a polyamideimide adhesive having a Tg of 225° C., a waterabsorption of 1.8% by weight and a broadening length of 2.0 mm, toproduce a composite film of a total thickness of 59 μm.

A lead frame with composite film attached was produced in the samemanner as in Example 1 with the exception that the composite film madeas above was used in place of the composite film of Example 1. Noflashes were observed by microscopic examination of the edges of thepunched composite film.

EXAMPLE 4

Each side of a polyimide film (UPILEX-S) having an edge tearing strengthof 35 kg/20 mm and a thickness of 50 μm was coated with a 25 μm thicklayer of the same polyamideimide adhesive as that used in Example 3, toproduce a composite film of a total thickness of 100 μm.

A lead frame with composite film attached was produced in the samemanner as in Example 1 with the exception that the composite film madeas above was used in place of the composite film of Example 1. Noflashes were observed by microscopic examination of the edges of thepunched composite film.

EXAMPLE 5

Each side of a polyimide film (UPILEX-S) having an edge tearing strengthof 30 kg/20 mm and a thickness of 50 μm was coated with a 15 μm thicklayer of the same polyamideimide adhesive as that used in Example 3, toproduce a composite film of a total thickness of 80 μm.

A lead frame with composite film attached was produced in the samemanner as in Example 1 with the exception that the composite film madeas above was used in place of the composite film of Example 1. Noflashes were observed by microscopic examination of the edges of thepunched composite film.

COMPARATIVE EXAMPLE 2

Each side of a polyimide film (UPILEX-S) having an edge tearing strengthof 25 kg/20 mm and a thickness of 25 μm was coated with a 20 μm thicklayer of the same polyamideimide adhesive as that used in Example 1, toproduce a composite film of a total thickness of 65 μm.

A lead frame with composite film attached was produced in the samemanner as in Example 1 with the exception that the composite film madeas above was used in place of the composite film of Example 1. A greatmany flashes were observed by microscopic examination of the edges ofthe punched composite film.

COMPARATIVE EXAMPLE 3

A composite film having a total thickness of 65 μm was produced in thesame manner as in Comparative Example 2 with the exception that apolyimide film (UPILEX-S) having an edge tearing strength of 30 kg/20 mmand a thickness of 25 μm was used.

A lead frame with composite film attached was produced in the samemanner as in Example 1 with the exception that the composite film madeas above was used in place of the composite film of Example 1. Manyflashes were observed by microscopic examination of the edges of thepunched composite film.

EXAMPLE 6

Each side of a polyimide film (UPILEX-S) having an edge tearing strengthof 25 kg/20 mm and a thickness of 25 μm was coated with a 12 μm thicklayer of the same polyamideimide adhesive as that used in Example 1, toproduce a composite film of a total thickness of 49 μm.

A lead frame with composite film attached was produced in the samemanner as in Example 1 with the exception that the composite film madeas above was used in place of the composite film of Example 1. Noflashes were observed by microscopic examination of the edges of thepunched composite film.

EXAMPLE 7

Each side of a polyimide film (UPILEX-S) having an edge tearing strengthof 30 kg/20 mm and a thickness of 25 μm was coated with a 15 μm thicklayer of the same polyamideimide adhesive as that used in Example 1, toproduce a composite film of a total thickness of 55 μm.

A lead frame with composite film attached was produced in the samemanner as in Example 1 with the exception that the composite film madeas above was used in place of the composite film of Example 1. Noflashes were observed by microscopic examination of the edges of thepunched composite film.

COMPARATIVE EXAMPLE 4

A composite film having a total thickness of 55 μm was produced in thesame manner as in Example 7 with the exception that a polyimide film(UPILEX-S) having an edge tearing strength of 20 kg/20 mm and athickness of 25 μm was used.

A lead frame with composite film attached was produced in the samemanner as in Example 1 with the exception that the composite film madeas above was used in place of the composite film of Example 1. Flasheswere observed by microscopic examination of the edges of the punchedcomposite film.

The values of T, R,(0.6T-8) and A/B in the above-described Examples andComparative Examples are shown in Table 1.

TABLE 1 T R A B A/B (μm) (kg/20 mm) 0.6T-8 (μm) (μm) (0.5-1.4) Example 159 35 27.4 34 25 1.36 Comp. 59 25 27.4 34 25 1.36 Example 1 Example 2100  30 28 50 50 1.00 Example 3 59 30 27.4 34 25 1.36 Example 4 100  3528 50 50 1.00 Example 5 80 30 28 30 50 0.60 Comp. 65 25 28 40 25 1.60Example 2 Comp. 65 30 28 40 25 1.60 Example 3 Example 6 49 25 21.4 24 250.96 Example 7 55 30 25 30 25 1.36 Comp. 55 20 25 30 25 1.36 Example 4

As evident from the results as shown in Table 1, the composite film ofthe present invention has such an excellent punching capability that noflashes are formed at the punched edges, and by using lead frames withthe composite film attached, extremely reliable semiconductor packagescan be produced.

What is claimed is:
 1. A method of producing a lead frame with acomposite film attached, comprising steps of: placing a composite filmover a lead frame, the composite film including a base film and anadhesive layer on at least one side of the base film, such that thecomposite film has the at least one adhesive layer, the composite filmhaving a thickness of T (μm), the base film having an edge tearingstrength of R (kg/20 mm), the at least one adhesive layer having a totalthickness of A, and the base film having a thickness of B, T beingrelated to R by a numerical formula R>0.6T-8 when T≦60, or by anumerical formula R≦28 when T>60, and A/B being 0.5 to 1.4, with oneadhesive layer facing the lead frame, wherein the at least one adhesivelayer is made of an adhesive having a broadening length of 2 mm or less;punching the composite film into a strip by using a punching metal mold;and pressing the strip to the lead frame with the punching metal mold ata pressure of 0.1 to 10 MPa for 0.1 to 5 seconds.
 2. The method of claim1, wherein the lead frame is heated to 200° to 500° C. during saidpunching.
 3. The method of claim 1, wherein the adhesive layer is madeof an adhesive having a glass transition temperature of 150° to 350° C.and a water absorption of 3% or less.
 4. The method of claim 3, whereinthe base film is made of a material having a glass transitiontemperature higher than that of said adhesive, and having a waterabsorption of 3% or less.
 5. The method of claim 4, wherein the adhesiveis a polyimide adhesive or a polyamide adhesive.
 6. The method of claim4, wherein the base film is a heat resistant film made of a resinselected from the group consisting of polyimide, polyamide, polysulfone,polyphenylenesulfide, polyetheretherketone and polyarylate.
 7. Themethod of claim 4, wherein each side of the base film is coated with theadhesive layer, and the thickness A is the total thickness of theadhesive layers.
 8. The method of claim 4, wherein the base film has athickness of 5 to 150 μm, and each adhesive layer has a thickness of 5to 50 μm.
 9. The method of claim 4, wherein the glass transitiontemperature of the base film is at least 200° C.
 10. The method of claim1, wherein the broadening length is 0.5 mm or less.
 11. The method ofclaim 1, wherein the base film has a thickness of 20 to 125 μm, and eachadhesive layer has a thickness of 10 to 30 μm.
 12. The method of claim1, wherein R is 25 to
 35. 13. The method of claim 1, wherein T isrelated to R by a numerical formula 35≧R>0.6T-8 when T≦60, or by anumerical formula 35≧R>28 when T>60.
 14. A The method of claim 1,wherein T≦60.
 15. A method of bonding a semiconductor chip to a leadframe, comprising steps of: punching a composite film to punch out astrip of the composite film, the composite film including a base filmand an adhesive layer on each side of the base film, the composite filmhaving a thickness of T (μm), the base film having an edge tearingstrength of R (kg/20 mm), the adhesive layer having a total thickness ofA, and the base film having a thickness of B, T being related to R by anumerical formula R>0.6T-8 when T≦60, or by a numerical formula R≧28when T>60, and A/B being 0.5 to 1.4, and wherein the adhesive layer ismade of an adhesive having a broadening length of 2 mm or less; applyingthe strip to the lead frame by pressing the strip so that one adhesivelayer of the strip contacts the lead frame; and bonding a semiconductorchip onto another adhesive layer of the strip.
 16. The method of claim15, wherein the composite film is placed over the lead frame, with oneadhesive layer facing the lead frame body, and punched with a punchingmetal mold to punch out the strip of the composite film, and the stripis pressed to the lead frame with the punching metal mold at a pressureof 0.1 to 10 MPa for 0.1 to 5 seconds.
 17. The method of claim 15,wherein the lead frame is heated to 200° to 500° C. during saidpunching.
 18. The method of claim 15, wherein the adhesive layer is madeof an adhesive having a glass transition temperature of 150° to 350° C.and a water absorption of 3% or less.
 19. The method of claim 15,wherein the base film is made of a material having a glass transitiontemperature higher than that of said adhesive, and having a waterabsorption of 3% or less.
 20. The method of claim 15, wherein theadhesive is a polyimide adhesive or a polyamide adhesive.
 21. The methodof claim 15, wherein the base film is a heat resistant film made of aresin selected from the group consisting of polyimide, polyamide,polysulfone, polyphenylenesulfide, polyetheretherketone and polyarylate.22. The method of claim 15, wherein the base film has a thickness of 5to 150 μm, and each adhesive layer has a thickness of 5 to 50 μm. 23.The method of claim 15, wherein the glass transition temperature of thebase film is at least 200° C.
 24. The method of claim 15, wherein R is25 to
 35. 25. The method of claim 15, wherein T is related to R by anumerical formula 35≧R>0.6T-8 when T≦60, or by a numerical formula35≧R>28 when T>60.
 26. The method of claim 15, wherein T≦60.
 27. Acomposite film, comprising a base film and at least one adhesive layeron at least one side of the base film, the composite film having athickness of T (μm), the base film having an edge tearing strength of R(kg/20 mm), the at least one adhesive layer having a total thickness ofA, the base film having a thickness of B, T being related to R by anumerical formula R>0.6T-8 when T≦60 or by a numerical formula R≧28 whenT>60, and A/B being 0.5 to 1.4, wherein the at least one adhesive layeris made of an adhesive having a broadening length of 2 mm or less. 28.The composite film of claim 27, wherein T is related to R by a numericalformula 35≧R>R0.6T-8 when T≦60, or by a numerical formula 35R>28 whenT>60.
 29. The composite film of claim 27, wherein T≦60.
 30. Asemiconductor package containing the composite film of claim 27 as abonding element.
 31. The semiconductor package of claim 30, wherein T isrelated to R by a numerical formula 35≧R>0.6T-8 when T≦60, or by anumerical formula 35≦R>28 when T>60.
 32. The semiconductor package ofclaim 30, wherein T≦60.
 33. A lead frame with a composite film attached,comprising a lead frame body and a strip of a composite film applied tothe lead frame body, wherein the strip is punched out from the compositefilm, the composite film comprising a base film and at least oneadhesive layer on at least one side of the base film, the composite filmhaving a thickness of T (μm), the base film having an edge tearingstrength of R (kg/20 mm), the at least one adhesive layer having a totalthickness of A, the base film having a thickness of B, T being relatedto R by a numerical formula R>0.6T-8 when T≦60 or by a numerical formulaR≧28 when T>60, and A/B being 0.5 to 1.4, wherein the at least oneadhesive layer is made of an adhesive having a broadening length of 2 mmor less.
 34. The lead frame with a composite film attached of claim 33,wherein the lead frame body and the composite film are bonded to eachother by one adhesive layer of the composite film.
 35. The lead framewith a composite film attached of claim 33, wherein T is related to R bya numerical formula 35≧R>0.6T-8 when T≦60, or by a numerical formula35≧R>28 when T>60.
 36. The lead frame with a composite film attached ofclaim 33, wherein T≦60.
 37. A semiconductor package containing the leadframe of claim
 33. 38. The semiconductor package of claim 37, whereinthe lead frame body is bonded to a chip by the strip of the compositefilm applied to the lead frame body.
 39. The semiconductor package ofclaim 38, wherein the lead frame body is bonded to a heat spreader bythe strip of the composite film applied to the lead frame body.
 40. Acomposite film, comprising a base film and a least one adhesive layer onat least one side of the base film, the composite film having athickness of T (μm), the base film having an edge tearing strength of R(kg/20 mm), the at least one adhesive layer having a total thickness ofA, the base film having a thickness of B, T being related to R by anumerical formula R>0.6T-8 when T≦60, or by a numerical formula R≧28when T>60, and A/B being 0.5 to 1.4, wherein the at least one adhesivelayer is made of an adhesive having a broadening length of 2 mm or less,and wherein the composite film has a property that flashings can beprevented when punching out a portion of the composite film.